chore: remove sample tts_synthesize_text_file

[aliciawilliams]

Description

Removing unused sample with region tag 'tts_synthesize_text_file' and associated tests. Internal: b/345843637

Linter also picked up a few negligible updates.

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• [X] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [ ] README is updated to include all relevant information
• [ ] Tests pass: nox -s py-3.9 (see Test Environment Setup)
• [X] Lint pass: nox -s lint (see Test Environment Setup)
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [X] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to delete 2 region tags.

- [texttospeech/snippets/synthesize_file.py:28](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/68700f05fefc8dbaf5d2d5671fa78ed86fea8685/texttospeech/snippets/synthesize_file.py#L28), tag `tts_synthesize_text_file` - [texttospeech/snippets/synthesize_file.py:62](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/68700f05fefc8dbaf5d2d5671fa78ed86fea8685/texttospeech/snippets/synthesize_file.py#L62), tag `tts_synthesize_ssml_file`

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